In describing the position of objects on a page, objects are often superimposed over a background color. In the area a foreground object will occupy, the background color is removed leaving a knockout. A typical case is black text placed on top of a colored background. Since the black character is in the foreground, the background color behind it is knocked out. When printed, misalignment of the black plane can shift the character relative to its background knockout and allow the bare media sheet to show through the gap. This produces objectionable outlines around one or more sides of the character and even reduces legibility. Color trapping is a technique used to minimize the show through and may include spreading and choking. Spreading refers to expanding regions of a particular color beyond its normal boundaries. Choking refers to contracting a color region so that a small overlap exists between graphical regions where misregistration may occur.
Prior art patents disclose a color trapping technique that performs color trapping after the page was completely rendered. The methods include searching for boundaries between the various colors and using various trapping techniques. This is referred to as backend color trapping as the page is initially formed and then modified via color trapping. Backend color trapping requires an extra step because the color trapping is determined based on the initially formed page.
Prior art methods also trap different object types in the same manner. Therefore, a character is trapped in the same manner as other objects. The trapping simply identifies the boundaries between color and traps with one type of predetermined logic. This broad-brush approach may cause significant color shifts and irregularities in the halftone screens.